Several composite reverse osmosis membranes have been proposed, and such a conventional composite reverse osmosis membrane is produced by forming a thin film having a substantially selective separability on a porous support. This reverse osmosis membrane has a different structure from that of an asymmetric reverse osmosis membrane.
Several composite reverse osmosis membranes, each having been formed on a support as a thin film comprising a polyamide obtained by an interfacial polymerization of a polyfunctional aromatic amine and polyfunctional aromatic acid halide, are disclosed in e.g., JP-A-55-147106, JP-A-62-121603, JP-A-63-218208, and JP-A-2-187135. The term "JP-A" means an "unexamined published Japanese patent application". Other prior art references disclose composite reverse osmosis membranes wherein thin films comprising polyamide are formed on supports, and the polyamide is obtained by an interfacial polymerization of polyfunctional aromatic amine and polyfunctional alicyclic acid halide, for example, JP-A-61-42308. In general, an activated layer of a composite reverse osmosis membrane comprises crosslinked polyamide, since it can be easily produced. Such an activated layer has negative fixed charge groups. As a result, the anion removal rate is high but the cation removal rate is low in a high pH region when a desalination of mineral salts is conducted in a low-concentration region. Therefore, the overall performance of the membrane is decreased. In order to solve such problems, a composite reverse osmosis membrane wherein an activated layer is coated with organic polymers having positive fixed charge groups is disclosed in JP-A-62-266103.
The composite reverse osmosis membranes described above have a high desalination performance and a high water permeability, but it has further been desired for those membranes to improve the water permeability while keeping the high desalination performance from the standpoint of the efficiency. For these requirements, various kinds of additives are proposed e.g., in JP-A-63-12310. However, in the conventional composite reverse osmosis membranes, the improvement of the water permeability is still insufficient even where these additives are used, and a composite reverse osmosis membrane having a higher water permeability has been desired. The composite osmosis membrane disclosed in JP-A-62-266103 is manufactured to function as an adsorptive film. Thus the predetermined efficiency will be deteriorated because of some degrees of loss of the film after the membrane is used repeatedly. This membrane is especially inadequate for a second membrane to be used in a two-stage reverse osmosis treatment conducted before preparing ultra-pure water for manufacturing semiconductors. In this newly-developed treatment, a negatively-charged membrane having high desalination is used in the first stage, and the permeated liquid is supplied for the second stage. Thus, the desalination efficiency is deteriorated if the two membranes have the same performance. In addition, the membranes are coated with crosslinked layers of organic polymers having positively fixed charged groups, so the permeation flux is deteriorated. It is an economical disadvantage, so a membrane with high permeation flux is needed.